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Fix lock-order inversion between the runtime lock and the selector table lock #394

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Merged
davidchisnall merged 1 commit into from
Jul 1, 2026
+12 −0
Merged

Fix lock-order inversion between the runtime lock and the selector table lock #394

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12 changes: 12 additions & 0 deletions selector_table.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -447,6 +447,15 @@ extern "C" PRIVATE SEL objc_register_selector(SEL aSel)
return registered;
}
assert(!(aSel->types && (strstr(aSel->types, "@\"") != nullptr)));
// Acquire the runtime lock before the selector table lock. Registering a
// new selector resizes the dtables (objc_resize_dtables), which takes the
// runtime lock, while the class loader (__objc_load) takes the runtime lock
// first and then registers selectors. Acquiring the two in a consistent
// runtime-before-table order avoids a lock-order inversion
// (gnustep/libobjc2#391). Holding the runtime lock for the whole
// registration also keeps the new selector's index (the selector_list size)
// and the matching dtable resize atomic with respect to other resizes.
LOCK_RUNTIME_FOR_SCOPE();
LockGuard g{selector_table_lock};
register_selector_locked(aSel);
return aSel;
Expand All @@ -463,6 +472,9 @@ SEL objc_register_selector_copy(UnregisteredSelector &aSel, BOOL copyArgs)
{
return copy;
}
// Runtime lock before the selector table lock, for the duration of the
// registration; see objc_register_selector above and gnustep/libobjc2#391.
LOCK_RUNTIME_FOR_SCOPE();
LockGuard g{selector_table_lock};
copy = selector_lookup(aSel.name, aSel.types);
if (nullptr != copy && selector_identical(aSel, copy))
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